Although the R-T-B (R represents one or more kinds of rare earth elements and T represents one or more kinds of transition metal elements with Fe or the combination of Fe and Co as the necessity) based sintered magnet shows excellent magnetic properties, it tends to have a had corrosion resistance due to an inclusion of the rare earth element as the main component which is easily oxidized.
Therefore, in order to improve the corrosion resistance of the R-T-B based sintered magnet, the surface of the magnet body is generally treated by, for example, resin coating, plating or the like. On the other hand, the additive elements or the internal structure of the magnet body can be changed to improve the corrosion resistance of the magnet body itself. The improvement of the corrosion resistance in the magnet body itself is extremely important to heighten the reliability of a product that has been subjected to a surface treatment. Besides, said improvement may thereby allow a surface treatment simpler than the resin coating or plating, which is advantageous for reducing the cost of the product.
Conventionally, for instance, patent document 1 has suggested a technique to improve the corrosion resistance of a magnet by reducing the carbon content in a permanent magnet alloy to 0.4 mass % or less and inhibiting the intermetallics R—C of the rare earth element and carbon in a non-magnetic R-rich phase to 1.0 mass % or less. In addition, patent document 2 has proposed a technique to improve the corrosion resistance by setting the concentration of Co in an R-rich phase at 5 mass % to 12 mass %.
However, in the conventionally used R-T-B based sintered magnet, water such as water vapor in the environment oxidizes R in the R-T-B based sintered magnet to generate hydrogen. Then, the R-rich phase in grain boundary absorbs the hydrogen, resulting in corrosion of the R-rich phase. In this way, the magnetic properties deteriorate in the R-T-B based sintered magnet.
In addition, as suggested in patent document 1, in order to reduce the carbon content in the magnet alloy to 0.04 mass % or less, it is necessary to greatly reduce the amount of the lubricant added to improve the magnetic field orientation when the magnet is pressed in a magnetic field. Therefore, the orientation degree of the magnetic powders in a green compact decreases and the residual magnetic flux density Br after sintering also decreases so that it is not possible to obtain a magnet having sufficient magnetic properties.
On the other band, as suggested in patent document 2, it is necessary to increase the content of Co in the raw material so as to increase the Co concentration in an R-rich phase. However, since Co also enters the main phase of R2T14B phase to substitute Fe, it cannot be achieved to increase the Co concentration only in the R-rich phase. More Co should be added than that as needed in the R-rich phase. Therefore, production cost rises as the amount of the expensive Co in use increases, and magnetic properties deteriorate as Fe in the main phase is substituted with Co at a level more than that as needed.